A modern earth boring drilling assembly typically includes a drill string with a drill bit at the base. As the drill bit advances though the earth, material being cut by the bit needs to be removed. A drilling fluid supplied through the drill string travels through the drill bit and across its face to flush material away from the face of the drill bit, into the annular passage between the drill string and the wall of the hole toward the opening of the hole. The drill fluid may be gas, such as air, or liquid, such as drilling mud.
If gas is utilized as the drilling fluid, the gas may be utilized in a positive displacement motor (PDM) to impart rotation on the drill bit at the bottom of the hole. Because gas is compressible, rotation speed of the PDM may vary in response to torque, even with a constant gas flow rate through the PDM. As drilling is carried out, pressure is imparted to the drill bit through the drill string. This may lead to situations in which the bit, whether a hammer, fixed cutter, or roller cone bit, may spin faster when the drilling pressure is removed during a pause drilling. This has may cause whipping contact with the wall of the borehole, potentially damaging the bit and/or the PDM.
The rapid spinning is caused by increased air flow through the down-the-hole (DTH) hammer when it is lifted off the bottom of the hole as opposed to during drilling. Along these lines, when a DTH hammer is lifted off the bottom of the hole, the bit is allowed to drop about 1-2″ out of the hammer. This shifts ports inside the tool into a separate mode of operation, in which the impact cycle stops and air that would otherwise move the piston exits directly through the hammer. This path is much less restrictive than the path during drilling. As a result, pressure inside the drill string rushes through the hammer when the string is lifted, resulting in a sudden increase in rotation speed.
Another purpose of drilling fluid is to help prevent influx of fluid from the formation being drilled into the hole. To address this situation, air utilized at the drilling is replaced by fluid, such as drilling mud, as the circulating fluid to stop the influx of fluids from the formation being drilled. This practice typically involves supplying high fluid flow rates to the drill string to establish an annular fluid column of sufficient weight to overcome the pressure of formation fluids. This necessitates opening of alternative flow passages between the drill string and annulus.
Flow of drilling fluid may create damaging or suboptimal drilling conditions in other circumstances when utilizing a PDM as well as with utilizing other types of drilling equipment. For example, in certain situations with rotary or DTH blasthole drilling, the flow capacity of the air compressor supplying drilling air is greater than can be supplied at a minimum required pressure at the hammer or bit to support the drilling process. In such situations, when the compressor reaches a maximum rated pressure, the compressor throttles back flow output. As a result, the compressor produces less than a potential flow due to the restriction introduced by the hammer or bit.
Additionally, fixed-orifice drill string flow elements including a series of holes arranged at an upward angle in the annulus between the hole and the drill string may be utilized to help create a flow out of the hole to remove cuttings and debris. Utilizing such flow elements may result in a vacuum effect below the device, scavenging flow away from the drilling face of the bit. This effect has been shown to be so powerful that it may accelerate abrasive wear on external surfaces of hammers and bits.
Typically, alternate flow passages are provided utilizing pump-out sub-assemblies, or subs, that may utilize extremely high pressure or external impacts (shear pin) to open the secondary passage to annulus. If extremely high pressure is utilized, a rupture disc may be utilized. The rupture disc is designed to provide a leak-tight seal within a pipe or vessel until the internal pressure rises to a predetermined level. At that point the rupture disc bursts preventing damage to the equipment from overpressure. The shear pin will break if the rotation becomes too great. However, once employed the rupture disc and shear pin must be replaced before providing the functionality again.
There is currently no system on the market to actively vary flow between on and off bottom conditions. Currently available pump-out subs require either extreme fluid pressures or introduction of a steel bar to the drill string. This not only complicates operation, but makes a response to rapid formation fluid influx time consuming.